It’s needed any time 2 or more gods are building tanky in any match and/or are building magical protection aura items like Heartward, Runic, Gauntlet. The only time you wouldn’t pick it up is either if there was no tanks in the match and the squishies built no magical protection items or if the only tank in the match was useless (I.e not peeling for their teammates or pressuring/damaging any squishy gods).

I might say, though, that even if 2 enemies are tanky, Ob Shard isn't "needed," just preferred if you want to do significant damage to those tanks as well. Otherwise, if you mainly target squishies with your normal kit, you'll probably do more damage with another offensive item...Ob Shard's power isn't that high, and the pen equivalence against a squishy god with 48 magical protection is measly...below 10.

Keep in mind, when you use your ult, you're getting their kit. When others build their pen, you'll get their items, so you can unleash on tanks with those gods if you want.

SmiteFire & Smite

Smite is an online battleground between mythical gods. Players choose from a selection of gods, join session-based arena combat and use custom powers and team tactics against other players and minions. Smite is inspired by Defense of the Ancients (DotA) but instead of being above the action, the third-person camera brings you right into the combat. And, instead of clicking a map, you use WASD to move, dodge, and fight your way through the detailed graphics of SMITE's battlegrounds.